1. Field of the Invention
The present invention relates to a resistance welding apparatus for supplying a welding current between two electrodes mounted on respective gun arms to weld workpieces clamped between the electrodes.
Resistance welding apparatus are widely employed to weld workpieces in shops and factories. A resistance welding apparatus has a pair of electrodes movable toward and away from each other. The electrodes which clamp workpieces therebetween are supplied with a welding current to fuse and weld the workpieces to each other.
2. Description of Background Art
One resistance welding apparatus includes an apparatus housing floatingly supported by a support bracket on which a transformer is fixedly mounted. The transformer has a secondary winding whose ends are connected to ends of terminal plates. The other ends of the terminals are electrically connected to two gun arms supporting respective electrodes through flexible copper plates. The flexible copper plates however present a contact resistance which results in an electric power loss and impose limitations on the direction in which the apparatus housing can be installed. The resistance welding apparatus is also heavy and large since the transformer is fixed to the support bracket through dedicated holders.
There has been proposed a resistance welding apparatus which includes an apparatus housing floatingly supported by a support bracket and a transformer fixedly mounted on the apparatus housing, with one terminal plate of the transformer being directly coupled to one gun arm (see Japanese Laid-Open Utility Model Publication No. 60(1985)-141977). The proposed structure reduces any electric power loss and the overall weight of the resistance welding apparatus, and also allows the apparatus housing to be installed in any desired direction.
Efforts have been made in recent years to mount a resistance welding apparatus on robot arms for automatizing welding operation. Therefore, there are demands for smaller and lighter resistance welding apparatuses.
Other resistance welding apparatuses which find wise use today include first conductive plates connected respectively to the ends of the secondary winding of the transformer, rectifiers and second conductive plates attached to the first conductive plates in superposed fashion, and gun arms connected to the second conductive plates through respective flexible copper plates.
However, a large electric power loss is caused by joints between the second conductive plates connected to the rectifiers and the flexible copper plates and also between the flexible copper plates and the gun arms, and also by the internal resistance of the flexible copper plates. If a relative large welding current is required as when welding thick plates or plated steel plates in particular, then since a large electric power loss would result from the contact resistance and the internal resistance, a large welding current cannot be supplied efficiently to the electrodes.
At the time a relatively large current is supplied to weld thick plates or plated steel plates, the electrodes and the rectifiers themselves are heated to a considerably high temperature. Accordingly, various cooling means are employed to cool the electrodes and the rectifiers in order to avoid adverse effects which such a high temperature would have on the electrodes and the rectifiers.
The cooling means are generally used exclusively to cool the electrodes or the rectifiers. Consequently, as many cooling means as the number of the electrodes and the rectifiers to be cooled must be employed in reality. Since such a number of cooling means need to be incorporated in the resistance welding apparatus, the resistance welding apparatus is large and heavy and highly costly.
In an attempt to reduce the size of a transformer in a resistance welding apparatus, it has been proposed to use a toroidal core as a transformer core and also to design a secondary winding as a container.
FIG. 1 of the accompanying drawings shows a conventional transformer 2 disclosed in Japanese Patent Publication No. 52(1977)-208, the transformer 2 having a secondary winding doubling as a container. The transformer 2 includes a hollow outer conductor 4, a bottom conductor 8 fixed to the lower end of the outer conductor 4, and a rodshaped inner conductor 10 extending upwardly from the center of the bottom conductor 8 and serving as a terminal conductor. A toroidal core 14 with a primary winding 12 mounted thereon is disposed around the inner conductor 10 within the outer conductor 4. A cover conductor 18 is mounted on the upper end of the outer conductor 4 and has a hole 16 through which the inner conductor 10 extends. A rod-shaped terminal conductor 20 is fixed to the cover conductor 18. The cover conductor 20 has two holes (not shown) through which lead-out wires of the primary winding 12 extend out of the transformer container.
The electric circuit of the transformer 2 thus mechanically constructed is shown in FIG. 2. The primary winding 12 mounted on the toroidal core 14 has X turns, and the secondary winding which is composed of the inner conductor 10, the bottom conductor 8, the outer conductor 4, and the cover conductor 18 has a single turn. Therefore, the transformer 2 has a transformation ratio of X : 1.
The rectifier circuit of the resistance welding apparatus is normally a full-wave rectifier circuit for higher efficiency and smaller ripple. The electric circuit of the transformer 2 shown in FIG. 2 may be associated with a single-phase full-wave bridge rectifier circuit, as shown in FIG. 3, having rectifiers 22a through 22d connected in a bridge. The single-phase full-wave bridge rectifier circuit however has twice as many rectifiers as those of a singlephase full-wave center-tap rectifier circuit. Accordingly, the rectifier circuit of the transformer 2 is large in size and complex in construction.
The toroidal core 14 is made of silicon steel or ferrite. However, the loss caused by the toroidal core 14 is increased even if the transformer 2 is employed in a resistance welding apparatus of the inverter type in which a direct current is converted to a high-frequency alternating current that is supplied to the transformer so as to be lowered in voltage, and the alternating current is converted again back to a direct current by the rectifiers. The cross-sectional area of the toroidal core cannot therefore be reduced, and the transformer cannot be sufficiently reduced in size and weight.